We will use your email address only for sending you newsletters. Please see our Privacy Notice for details of your data protection rights.

In a meeting with Foreign Secretary Philip Hammond at his official residence in London, the US Secretary of State said American and Russian defence secretaries had held extensive talks by telephone on defeating ISIS.

Speaking after the meeting with the Foreign Secretary, Mr Kerry, who is on a three-day trip to the UK, said: "Would we welcome Russian help in going against Isil? Obviously. We have talked about it for some period of time."

He added: "But the other part of the equation is Assad and how you resolve the fact that he is a magnate for foreign fighters to come to the region."

Mr Hammond said the crisis in Syria is becoming "more complicated" by Russian involvement in tackling Islamic State and the possibility of British military action in Syria had been made difficult by Russian involvement.

He indicated there were reservations about any Russian involvement with them being a key ally of Bashar Assad's regime.

Mr Hammond said the Government had been clear that if it believed airstrikes in Syria were necessary it would go back to parliament.

He said: "We keep that situation under continuous review and we will continue to talk to our allies about the military situation in the round."

He added: "Because of the Russian engagement, the situation in Syria is becoming more complicated. I think we need to discuss this as part of a much bigger problem, the migration pressures, the humanitarian crisis in Syria as well as the need to defeat Isil."

The Foreign Secretary said Britain and the US were "completely aligned" on the need for Assad to go, but the timing and the way it happened would need to be discussed further.Mr Kerry said the US was looking at ways the United States could do more to help alleviate the refugee crisis in Europe that has been partly caused by the exodus of Syrians fleeing the dual terrors of Assad and Isil.

The US has already ploughed 4.1 billion dollars (£3.6bn) into support for Syrian refugees, he said. "We are committed to try to do more and we looking at exactly what it will take to do more," he added.